Laser Doppler Velocimeter Data Research Articles

Evolution of Gas Velocities Behind Flamelets in a Premixed Turbulent Bunsen Flame

In an earlier study of turbulent premixed Bunsen flames, we employed a two-component laser Doppler velocimeter (LDV) system and a three-element electrostatic probe to investigate influences of flamelets on local gas velocities in the reaction-sheet regime. Because the two-component LDV does not provide complete information about the gas velocity and the three-element probe does not provide complete information on the direction of flamelet motion, it was not possible to fully explain the results of the LDV measurements correctly. To remedy this difficulty so that the explanations can be completed, in the present work the two-component LDV is replaced by a three-component LDV and the three-element probe is replaced by a four-element probe, which provide full information on the direction of flamelet motion. The new LDV system was employed to measure the three components of velocity at a midflame point on the centerline of a propane–air flame of equivalence ratio 1.10 on a 26-mm-diameter Bunsen burner with fully developed turbulent pipe flow upstream at a Reynolds number of 6700. The LDV measurement volume was 1 mm below the leading electrode of the four-element electrostatic probe that provided the velocity and direction of motion of flamelets in the turbulent flame brush. It was found that when the coordinate system was rotated about a vertical axis to the plane containing the flamelet velocity vector, most of the burnt-gas motion following flamelet passage often remained largely in this plane, although in many cases a nearly constant rate of rotation about this plane was detected. The sequence of LDV data for gas motion in the burnt gas consistently showed movement beginning first in a direction different from that of flamelet passage, then gradually rotating until finally becoming the direction of flamelet passage. This behavior is explained qualitatively by considerations of momentum conservation and of the nature of the LDV measurements. Earlier speculations concerning the role of buoyancy are thereby resolved, now showing that buoyancy is not the principle controlling phenomenon, and additional information about turbulence structures in these flames is obtained.

Response of retinal blood flow to systemic hyperoxia as measured with dual‐beam bidirectional Doppler Fourier‐domain optical coherence tomography

AbstractPurpose Much effort was directed towards the quantification of retinal blood flow in the past decades. In the recent years, optical coherence tomography (OCT) based systems were introduced to measure retinal perfusion. The current study compares a new technique, dual beam bidirectional Doppler Fourier domain OCT (FD‐OCT), with laser Doppler velocimeter (LDV), while breathing ambient air and pure oxygen.Methods 10 healthy volunteers were included into the present study and two study days were scheduled. On one study day assessment of the effect of 100% O2 breathing on retinal blood velocities was performed using FD‐OCT. LDV was used to investigate retinal blood velocities on the other study day. In addition to velocity measurements retinal vessel diameters were assessed with Dynamic Vessel Analyzer (DVA, Imedos, Germany). Baseline data as well as hyperoxia data were compared between each study day.Results Reduced vessel diameters, velocities and blood flow were found while breathing 100% O2 compared to baseline. Our findings showed a high correlation between retinal blood velocities obtained with the OCT system and the LDV during baseline and under hyperoxia (p<0.01 each). However, velocities measured with the OCT were slightly higher.Conclusion Good correlation was found between dual‐beam bidirectional Doppler FD‐OCT and LDV data. Dual‐beam bidirectional Doppler FD‐OCT is a promising approach for studying retinal blood flow in vivo.

Resonant geometries for circulation pattern macroinstabilities in a stirred tank

AbstractCirculation pattern macroinstabilities and precessing vortices are known to exist in stirred tanks, but until recently they have eluded a full quantitative analysis. In this article, frequency analysis of the axial velocity reveals a resonant frequency and geometry for the 45° pitched‐blade turbine (PBT) in a flat‐bottom cylindrical tank. Unlike other axial impellers, such as the hydrofoils A310 and HE3, the PBT produces a large‐scale circulation macroinstability at the scale of the vessel diameter. The macroinstability is superimposed on the turbulent random fluctuations with a timescale that is very long compared to the blade passage frequency and the inertial convective range of the turbulent cascade. In this article it is shown that the circulation pattern macroinstabilities generated by the 45° PBT are coherent and propagate throughout the tank only under very specific conditions (D = T/2 and C/D = 0.50). The normalized frequency of the coherent macroinstability, or Strouhal number, is fMI/N = 0.186. This corresponds to a period of roughly five rotations of the impeller, or 20 individual blade passages. The laser Doppler velocimeter data used in this work are unevenly spaced because they are collected in burst detection mode. This severely complicates spectral or frequency analysis. Standard algorithms such as the fast Fourier transform, autocorrelation methods, and wavelet analysis all require evenly spaced data. The Lomb periodogram was successfully used to extract the low‐frequency content of the unevenly spaced data with a higher accuracy than is possible with other methods. The Lomb method also eliminates a short time‐averaging step that was required in earlier work. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2986–3005, 2004

Study of Vortical Separation from Three-Dimensional Symmetric Bumps

Surface mean pressures, oilflow visualization, and three-velocity-component laser Doppler velocimeter (LDV) measurements are presented for a turbulent boundary layer of momentum thickness Reynolds number, Re θ 7300, and of thickness 6 over two axisymmetric bumps of height H = 6 and 26 and one symmetric bump of H = 26. LDV data were obtained at one plane, x/H ≥ 3.26, for each case. Complex vortical separations occur on the lee side and merge into large streamwise mean vortices downstream for the two axisymmetric cases. The near-wall flow (y + < 90) is dominated by the wall. For the axisymmetric cases, the vortices in the outer region produce large turbulence levels near the centerline and appear to have low-frequency motions that contribute to turbulent diffusion. For the case with a narrower spanwise shape, there are sharper separation lines and lower turbulence intensities in the vortical downstream flow

Investigation of Stator-Rotor Interaction in a Transonic Turbine Stage Using Laser Doppler Velocimetry and Pneumatic Probes

The current paper presents steady and unsteady flow data of a transonic test turbine stage operating under flow conditions similar to modern highly loaded gas turbines. Measurements were performed between stator and rotor as well as downstream of the rotor in planes perpendicular to the rotor axis. Time-resolved axial and tangential velocities were measured by a two-component laser doppler velocimeter (LDV) to investigate unsteady phenomena, while time-averaged flow properties were measured by means of a pneumatic seven-hole probe for all three spatial directions. The time-resolved investigation done by LDV allows to present velocity fields, flow angles and turbulence data at different stator-rotor positions during one blade passing period. Averaging these results enabled comparison with the pneumatic multihole probe measurement. LDV data and stage geometry can be obtained per email request and used for computational fluid dynamics (CFD) code verification.

Analog Sensor Design Proposal for Laser Doppler Velocimetry

Laser Doppler velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, in most applications, i.e., in industrial processes, such a system is often too expensive. This paper discusses a technique based on the use of an analog phase-locked loop and an analog integrator system for processing laser Doppler velocimeter data to infer particle velocity. This method appears to be suitable for designing low-cost integrated LDV sensors. A SIMULINK program has been written in order to validate the method for velocities in the 10-80 mm/s range. Finally, the performance of the estimator is illustrated by Monte-Carlo simulations obtained from synthesized Doppler signal.

Velocity profiles in a closed, unbaffled vessel: comparison between experimental LDV data and numerical CFD predictions

The velocity profiles and the turbulent kinetic energy distribution were obtained for the flow generated by a 6-blade, 45° pitched-blade turbine in an unbaffled, flat-bottom, cylindrical tank provided with a lid, and completely filled with water. The mean and fluctuating velocities in all three directions were experimentally measured with a laser-Doppler velocimeter (LDV) at five different heights and twenty radial positions within the vessel. A computational fluid dynamic (CFD) software package ( FLUENT) was used to numerically predict the velocity distribution, fluctuating velocities, power consumption, and pumping capacity of the impeller. Turbulence effects were simulated using either the k− ε model or the algebraic stress model (ASM). The experimentally obtained mean velocities and turbulent kinetic energies on the top and bottom horizontal surfaces of the region swept by the impeller were used as boundary conditions in the simulations. The agreement between the experimental data and the numerical predictions was found to be significant in most cases. Velocity predictions based on ASM were found to be superior to those based on the k− ε model. In general, the tangential velocities were found to be significantly bigger than the other two velocity components. In the r− Z plane a strong radially oriented flow was observed to emerge from the impeller, producing two main recirculation flows, one above and the other below the impeller. The dimensionless mean velocities in all three components and the dimensionless turbulent kinetic energies were found to be nearly independent of the impeller rotational speed.

Evaluation of methods for assessing spectra from laser Doppler velocimetry signals

A critical evaluation is made of the spectral bias which occurs in the use of a laser Doppler velocimeter (LDV). In order to accommodate the highly turbulent random sampled LDV data, a doubly stochastic Poisson process is generated. Four well-known processing algorithms, i.e., the direct transform (RG), the transit-time weighting method (TW), the sample-and-hold method (SH), and the modified Shannon sampling technique (SR), are evaluated and compared with the theoretical spectra for various sampling and flow conditions. Assessment of these algorithms is made for varying data densities (0.05 ⩽ d.d. ⩽ 5.0) and turbulence levels (t.i. = 30% and 100%). Furthermore, the effects of the Reynolds stress coefficients and of the transversal standard deviations on the spectral bias are examined.

Supersonic near-wake afterbody boattailing effects on axisymmetric bodies

An experimental investigation of the near-wake flowfield downstream of a conical boattailed afterbody in supersonic flow is presented. The afterbody investigated is typical of those for conventional boattailed missiles and projectiles in unpowered flight. Flow visualization, mean static pressure measurements, and three-component laser Doppler velocimeter data have been obtained throughout the near wake of the body. The effects of afterbody boattailing on the physics of the near-wake flow are determined by comparing the present data with similar data obtained on a cylindrical afterbody. Results indicate that a net afterbody drag reduction of 21 % is achieved with the current boattailed afterbody for a freestream Mach number of 2.46. The shear-layer growth rate, and therefore mass entrainment from the recirculation region behind the base, is shown to be significantly reduced by afterbody boattailing due to the reduction in turbulence levels throughout the near wake as compared to the cylindrical afterbody.

Comparison of optical measurement techniques for turbomachinery flowfields

A preliminary set of measurements were made of the flowfield in the Purdue Research Centrifugal Compressor using a laser two-focus (L2F) velocimeter and a laser Doppler velocimeter (LDV). After a review of the preliminary results, the LDV system was chosen to continue this research due to the advantages it demonstrated over the L2F system in making measurements in this flowfield. The L2F data are compared and contrasted to the LDV data. While this comparison is not to insinuate the local features of the LDV data are universally correct, an evaluation of the global features of the compressor flowfield based upon the LDV measurements demonstrate consistency, i.e. measurements at various planes in the flowfield demonstrate conservation of mass and a reasonable distribution of work in the compressor. In addition, methodologies to determine the effect of the measurement volume geometry for the two systems are presented. Finally, the advantages and disadvantages of using the L2F system for turbomachinery flowfields is discussed in terms of measurement accuracy, applicability to general turbomachinery flowfields, and the capability to make measurements in regions of high noise due to stray reflections. Specific examples based upon this experimental work are presented. 12 refs.

Assessment of turbulent spectral bias in laser Doppler velocimetry

A critical evaluation is made of the spectral bias which occurs in the use of a laser Doppler velocimeter (LDV). In order to accommodate the randomly sampled LDV data, statistical treatments of particle arrival times are needed. This is modeled as a doubly stochastic Poisson process which includes the intensity function of the velocity field. Three processing algorithms are considered for spectral estimates: the sample and hold method (SH), the modified Shannon sampling technique (SR), and the direct transform (RG). Assessment is made of these for varying data densities (0.05 ≤ d.d ≤ 5) and turbulence levels (t.i.=30%, 100%). The effects of the values of the Reynolds stress coefficients and the transversal standard deviation on the spectral contents were examined. As an improved version of the spectral estimator, the utility of POCS (the projection onto convex sets) has been tested in the present study. This algorithm is found useful to be in the region when d.d. ≳ 3.

Spectral Analysis of Fluid Flow in Oscillating Grid Reactor

A methodology for quantifying flow-field characteristics from laser Doppler velocimeter (LDV) measurements was developed. This methodology was used to quantitatively investigate two mixing conditions (0.25 Hz and 0.75 Hz) in an oscillating grid reactor. The resulting time records of fluid velocity in the reactor verified that the fluid motion was significantly different for the two mixing conditions. A computer program was developed based on theoretical work of others to calculate the power spectral density function directly from the randomly sampled LDV data. These power spectra, calculated from velocity-time records, were used to quantify the length scales occurring in the reactor and their associated frequencies, the differences in the fluid motion associated with the two mixing conditions, and the differences in fluid motion with respect to measurement plane and measurement position in the reactor. Significant differences were observed in the larger-scale fluid motions and not in the smaller-scale (higher frequency) fluid motions.

U′v′ Reynolds stress in the viscous sublayer over a wide range of Re numbers

AbstractTo test the ability of different turbulence models to predict the turbulent momentum transport in the viscous sublayer, measurements are performed with a laser Doppler velocimeter (LDV). The main objective of the experiments is to measure the u′v′ correlation in the near wall region. The results support the correlation: −u′v′+ = Aky+3. The results of 2‐D computer simulations using four different low Reynolds number k‐ϵ turbulence models are compared with the experimental LDV data. The models tested are the original low Reynolds number k‐ϵ model of Jones and Launder (1972, 1973) and three later modifications by Chien (1980), Lam and Bremhorst (1981), and Nagano and Hishida (1987). The u′v′ correlation was found to be underestimated with all four models, but the Chien model was superior, with only a 40% underestimation in the viscous sublayer.

Application of particle image velocimetry in high-speed separated flows

A particle image velocimetry (PIV) system has been developed for use in high-speed separated air flows. The complete system was developed to improve the spatial resolution and accuracy of the PIV technique as applied in high-speed compressible flows and is the first to incorporate both birefringent image shifting and submicron seed particles for analysis of separated flowfields. The system has been proven in preliminary experiments using a simple low-speed round jet flow and has been validated for accuracy with both known displacement simulated PIV photographs and with uniform flow experiments at Mach 0.5 (170 m/s) for comparison with pressure and laser Doppler velocimeter data

Laser Doppler velocimeter measurements of thermal counterflow jet in He II

A thermal counterflow jet in stationary superfluid helium (He II) has been investigated using a laser Doppler velocimeter (LDV) with H 2-D 2 solid particles as tracer elements in the flow. The average normal fluid velocity measured with the LDV agrees well with a simple theoretical prediction below the condition of subcritical heating, where the velocity is small. For high velocities the measured velocity is less than that predicted theoretically. High resolution LDV data show the existence of regular oscillations in the velocity, superposed on a laminar base flow. The velocity oscillation is found to result from the second sound Helmholtz oscillation in the jet chamber. Supercritical heating generates quite a turbulent jet, in which the fluid dynamic behaviour seems very similar to that for fully developed turbulent jets in normal viscous fluids.

Structure of self-excited oscillations in transonic diffuser flows

Two-component laser Doppler velocimeter (LDV) measurements were made in a supercritical, separated, transonic diffuser flow exhibiting self-excited oscillations. The velocity data were ensemble-averaged with respect to the shock oscillation phase, and maps of various flow quantities were generated. The time evolution of the fluctuating velocity field shows a large, rotating structure that originates near the upstream edge of the separation bubble and is convected downstream. The streamwise velocity fluctuation pattern for the self-excited oscillations resembles the oscillation pattern that occurs when the flow is mechanically excited at the downstream end. Velocity fluctuations calculated from ensemble-averaged core total and static pressure data show good agreement with the LDV data.

Turbulence spectra from individual realization laser velocimetry data

Two techniques — a modified correlation method and a direct transform method — have been evaluated for use in making spectral estimates from randomly-sampled random data (such as turbulence data obtained with an individual realization laser Doppler velocimeter (LDV)). The effect of “bad” points (which usually appear randomly in actual LDV data sets) on the spectral estimates has been studied. Necessary modifications and extensions to the techniques have been determined based on studies using simulated data with known spectral characteristics. The direct transform method is found to have certain advantages over the correlation method.

Adaptive-wall wind-tunnel development for transonic testing

Experimental techniques for rapid assessment and correction of wall interference in an adaptive-wall wind tunnel are described. The experimental arrangement allows laser velocimetry measurements on two control surfaces and incorporates a dedicated computer for data processing. The apparatus and its instrumentation are described, and typical results from an experiment on a nonlifting NACA 0012 airfoil at M = 0.78 are discussed. It is concluded that the time to acquire laser Doppler velocimeter data should be decreased, and the possibility of using one-step algorithms should be investigated.

Anatomy of a turbulent spot

Measurements have been made in the plane of symmetry of a turbulent spot to study the issue of entrainment. The mean flow is assumed to be similar in coordinates (x/t, y/t), and laser Doppler velocimeter data at several stations are used to determine the effective origin in x and t and to establish particle trajectories with respect to the stationary interface. Entrainment occurs over most of the rear interface and also close to the wall at the front of the spot. Flow visualization has also been used to obtain additional information about entrainment and sublayer structure. Pictures of the underside of spots, wedges, and boundary layers were taken through a glass wall, using a very heavy concentration of aluminum flakes, so that only motions occurring for y+ less than about 30 were visible. Sublayer streaks have essentially the same streaky, knotted appearance in all three types of flow. Optical correlations confirm the value λ+∼100. An attempt has been made to estimate the strength of sublayer streaks as streamwise vortices, assuming that the streaks are a manifestation of local Taylor–Görtler instability. The results account for many of the phenomena actually observed in the sublayer.

Data acquisition system for laser Doppler velocimeters

Instrumentation for rapidly acquiring, storing, and processing laser Doppler velocimeter data is described. Electrical pulses having amplitudes that are inversely proportional to the Doppler frequencies are generated and then counted with a multichannel analyzer. Velocity distribution data may then be printed out, graphically displayed, or computer processed.